This invention relates to a method of removing coke deposited within gas passages of an apparatus for the thermal cracking of hydrocarbons.
For thermally cracking hydrocarbon oils such as crude oils, reduced crude oils and residual oils, it is known to use a dual tower type apparatus composed of heating and cracking towers each containing a fluidized bed of solid particles continuously recirculating between the two towers. In such an apparatus, the feedstock is fed to the cracking tower where it is subjected to thermal cracking conditions by contact with the heated, fluidized soild particles. The cracked product is withdrawn overhead from the cracking tower for recovery while the solid particles are introduced into the heating tower, where they are heat-regenerated by contact with a combustion gas introduced into the heating tower from a combustion furnace provided adjacent thereto. The thus heat-regenerated solid particles are then recycled to the cracking tower.
When the apparatus is operated for a certain period of time, for example for five weeks, there arises a need to conduct a decoking operation since a large amount of coke is deposited and accumulated within the gas passages downstream of the cracking tower, such as pipes, cyclone, quenching device, etc. In such a case, it has been the general practice to disjoint the apparatus for the removal of the coke deposited on the inside surfaces of the pipes, cyclone and other parts, the separated parts being assembled after being cleaned of coke. The decoking of the thus separated parts are generally effected physically using suitable cleaning devices and, thus, is very troublesome. Moreover, since the apparatus is generally high and heavy, the disjointing and assembling procedures themselves are also time-consuming. Therefore, the conventional decoking method is disadvantageous from an economic point of view.
There is known a so-called steam-air decoking method in which a hot combustion gas and steam are alternately allowed to flow through the gas passages of the apparatus so that the coke deposited within the gas passages is subjected to alternate heating and cooling, whereby the coke is spalled. The spalled coke pieces are carried with the stream of the high speed steam. This method does not require disjointing work. However, this method is applicable only to apparatuses of a small diameter cracking tube formed of a material of a large thermal expansion coefficient, such as a metal, since such an apparatus alone enables easy exfoliation of the coke from the surfaced of the tube and high speed flow of steam. The steam-air decoking method cannot be applied to the above-mentioned dual tower type cracking apparatus which is generally made of an inorganic refractory material and which is large in pipe diameter.